I want to get the current column letter

For example:

instead of

=AVERAGE(T1:T21)

I want

=AVERAGE(currentColumnLetter1:CurrentColumnLetter21)

Don’t really understand what is your real problem? Why can’t you write =AVERAGE(T1:T21) instead? If you would like to copy formula, then references will automatically change to current column/row. Please write more what are you trying to achieve. Also provide the version of LibreOffice and operating system you are using.